The present invention relates to a mass spectrometer and a method of mass spectrometry. The preferred embodiment relates to an ion guide and method of guiding ions.
Various separation techniques for separating ions are known and may be broadly divided into two different types. A first type involves performing separation or filtering from a continuous stream of analyte ions. A second type involves analyte ions being delivered in a focussed pulse or as a discontinuous stream of ions.
Devices or techniques which can separate or identify individual components from a continuous stream of analyte ions include RF quadrupole mass filters, magnetic sector mass spectrometers, electrostatic analysers, UV and fluorescence spectrometers and differential ion mobility spectrometers.
Devices or techniques which require discontinuous delivery of analyte ions to separate or identify individual components include gas chromatography, liquid chromatography, Time of Flight mass spectrometers, ion mobility spectrometers, FTICR mass spectrometers and RF and electrostatic ion traps.
In general, for devices or techniques which require discontinuous delivery of analyte ions a period of time must be allowed to elapse between each introduction of a pulse of analyte ions. Often this period of time represents the entire time required to complete the analysis of a prior pulse of analyte ions.
Considering techniques wherein analyte molecules are continuously ionised to produce a continuous beam of ions, such as Electrospray ionisation, a narrow pulse of ions may be generated by gating a small section of the ion beam into the mass spectrometer. The analytical performance of the device is inversely proportional to the width in time of the gated portion of the ion beam. For example, in IMS analysis a typical analysis time may be of the order of 10 ms and a gate width may be of the order of 100 μs. If the remaining incoming ion beam is lost during the 10 ms analytical time then the duty cycle for such a device will be of the order of 1%.
Several approaches have been developed to try to improve the duty cycle for devices such as Ion Mobility Spectrometers (“IMS”). Ions may, for example, be accumulated in an ion storage trap and the ion trap may be used to generate a pulse of ions for analysis. Ions arriving at the ion trap during the analysis time may be accumulated within the ion trap ready for the next analysis period. This method has been used enabling duty cycles approaching 100% to be obtained for ion traps, IMS analysis and Time of Flight analysis.
However, efficient ion trapping with RF confining fields is difficult at pressures approaching atmospheric pressure and also at very low pressures where ions do not lose kinetic energy rapidly due to collisions with background gas molecules. Commercial atmospheric pressure IMS devices do not employ either RF ion traps or RF ion guides.
A second approach to the problem of poor duty cycle is multiplexing wherein ions are pulsed into a mass spectrometer at a frequency faster that the frequency imposed by the total analysis time of the device. This produces complex spectra which need to be de-convolved based on knowledge of the initial gating frequency or encoding pattern. Hadamard transform Time of Flight is an example of this approach.
A third approach commonly employed for Time of Flight mass spectrometry is orthogonal extraction. A relatively large section of an ion beam is extracted orthogonally by application of a sudden voltage pulse. As the energy spread of the beam in the orthogonal direction is relatively low compared with the axial direction, very high resolution time of flight analysis can be realised with duty cycles of the order of 20%-40%.
For time of flight analysis orthogonal extraction of a continuous ion beam is performed at very low pressures to minimise collisions with background gas which would otherwise degrade the performance of the Time of Flight analyser and/or cause collisionally induced fragmentation.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.